Explain the Relative Atomic Mass (ram), symbol Ar, and the Relative Molecular Mass?

Solution:

a)
The relative atomic mass or weight is defined as the ratio of the average mass of the naturally occurring isotopes of a chemical element to 1/12 of the mass of Carbon-12 atom. In other words, carbon-12 is taken as reference and 1/12 of Carbon-12 atom = 1 atomic mass unit (amu) = 1.660x10-27 kg.

Since it is established by taking carbon-12 as reference, that is why it is called “relative atomic mass” and represented by the symbol Ar. The relative atomic mass is the one you find in many periodic tables of chemical elements.

Examples of relative atomic mass (amu): H: 1.008, Be: 9.012, F: 19.00, Ne: 20.18, I: 126.9, U: 238.0

N.B: Those values may appear in the periodic table of chemical elements as simply Atomic Mass

b)
The relative molecular (or formula unit in ionic compounds) mass or weight is calculated as the sum of relative atomic masses of all atoms that form the chemical formula or formula unit of the chemical species. It is represented by the symbol Mr.

Examples of relative molecular mass (amu):

  • H2O : 1.008 x 2 + 16.00 = 18.016 = 18
  • CaSO4: 40.8 + 32.07 + 4 x 16.00 = 136.15
  • SO4: 32.07 + 4 x 16.00 = 96.07

N.B: When the above figures are expressed in g, then they are called Molar Mass, Mm (the concept of Mole will be discussed later)

As you notice, the combination of atoms to form molecules or crystal lattice doesn’t change the atomic mass of individual atoms or ions that form the molecule or the crystal. This is because the combination of atoms and the formation of chemical bonds doesn’t change the identity and mass of atoms or ions involved.